In recent years, vehicles such as a hybrid vehicle and an electric vehicle that are equipped with an electric motor as a power source have been improved to increase a capacity of a power storage unit for improving drive performance such as acceleration performance and a continuous travel distance. A structure having a plurality of power storage devices has been proposed as means for increasing the capacity of the power storage unit.
Japanese Patent Laying-Open No. 2003-209969 has disclosed a power supply control system provided with a plurality of power supply stages. This power supply control system includes a plurality of power supply stages that are connected in parallel together and supply a DC power to at least one inverter. Each power supply 6stage includes a battery and a boost/buck DC-DC converter.
In this power supply control system, the plurality of power supply stages are controlled to maintain an output voltage for the inverter by uniformly charging and discharging a plurality of batteries that are included in respective power supply stages.
Generally, in a power storage device such as a secondary battery and a capacitor, a capacity lowers with lowering of a temperature so that a charge/discharge property deteriorates with it. In the hybrid vehicle and others, therefore, the temperature of the power storage device must be rapidly increased when the temperature of the power storage device is low after a vehicle system started. Particular, in the system having a plurality of power storage devices such as the power supply control system disclosed in aforementioned Japanese Patent Laying-Open No. 2003-209969, it is necessary to increase the temperature of the power storage device before the start of driving for sufficiently bringing out merits of the large capacity of power storage unit.
However, aforementioned Japanese Patent Laying-Open No. 2003-209969 has merely disclosed that the power storage device is operated to charge and discharge uniformly the plurality of batteries included in respective power supply stages, and has not particularly discussed a method for rapidly increasing the temperature of the plurality of power storage devices at a low temperature.